Conversion Program with Pass by Value and Pass by Reference

Hi, this site has been extremely helpful with my path down learning C++ but I have ran into an issue on one of my assignments. Below is my code I wrote to create a basic program for conversions. My assignment now is to modify the program so that each calculation is performed by a user defined function. At least one of the user defined functions will utilize the "pass by value" method and at least one using the "pass by reference" method.

I am a little confused on how to start this and how it should exactly look like. If anyone could please provide me an example of how I should start, or some basic syntax so I can grasp the concept of how I should do this I would greatly appreciate it!